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Abstract

Physical activity is positively related to both physical fitness and physical, social and mental health in adolescence as well as later in life. Physical inactivity is an established risk factor for diseases like cardiovascular disease, type 2-diabetes and cancer. Representative data are essential to assess and monitor physical activity and inactivity in populations in order to study prevalence and time trends, including estimating the impact of physical activity on health outcomes. Few longitudinal studies have analysed predictors associated with changes in physical activity levels in adolescents. Little is known about which effects different physical activity patterns followed from adolescence to early adulthood, might have on cardiometabolic risk factors and mental health in early adulthood.

Aims

The overall purpose of this thesis was to test the reliability and validity of self-reported physical activity questions to investigate physical activity behaviour in adolescence, and from adolescence to young adulthood. Furthermore, we wanted to study physical activity patterns and their relation to risk factors for health and morbidity in a Norwegian population

Methods

To validate the answers of two of the most frequently used physical activity questionnaires; the WHO, Health behaviour in Schoolchildren (HBSC) questionnaire and the International Physical Activity Questionnaire (IPAQ), short version, we compared them with more objective measurements; the ActiReg and VO2peak. Data was collected from 71 randomly selected adolescents aged 13-18 years old (paper 1). Data from two prospective longitudinal surveys in the Nord-Trøndelag Health Study were used to study predictors of physical activity patterns and their relation to subsequent health risk through adolescence (paper 2) and from adolescence to young adulthood (paper 3).

Data from 2348 adolescents and their parents who participated in the Nord-Trøndelag Health Study (HUNT2, including the concomitant youth part, Young-HUNT1, 1995-97) and at follow-up in Young-HUNT 2, performed in 2000-2001 formed the basis for studying factors that predicted changes in physical activity during adolescence. A total of 1869 individuals participating in Young-HUNT1 (1995-97), aged 13-19 years (baseline), and followed-up at HUNT 3 (2006-08), aged 23-31 were included. We examined physical activity patterns and subsequent cardiovascular disease (CVD) risk factors and mental health in young adulthood.

The participants completed a self-reported questionnaire and participated in clinical examinations. Blood samples were collected only in adults.

The reliability (intraclass correlation coefficient) for the WHO HBSC questionnaire was 0.71 for frequency and 0.73 for duration, with significant differences between genders. The validity (Spearman correlation coefficients) for both the WHO HBSC questionnaire and the IPAQ measured against VO2peak was fair. The WHO HBSC questionnaire measured against VO2peak for girls was acceptable. Both questionnaires, except the walking question in IPAQ, showed a low correlation with the objective activity measure; the ActiReg (physical activity level and total energy expenditure measured continuously for seven days).

Overweight, dissatisfaction with life and no active participation in sports at baseline, were significant predictors of relapse in physical activity among boys during adolescence. For girls, smoking, consumption of alcohol, low maternal education and physical inactivity, predicted relapsing and inactive maintaining of physical activity. A higher level of education and more physically active parents at baseline were associated with increased physical activity during adolescence for both genders.

Those who maintained physical activity from adolescence to young adulthood (active maintainers), had significantly lower heart rate in young adulthood, compared to all other physical activity patterns. Active maintaining men had significantly lower waist circumference than relapsers and inactive maintainers. When adjusted for age and gender, waist circumference, BMI, heart rate, diastolic blood pressure and HDL-cholesterol, significant differences were demonstrated comparing active maintainers to other physical activity patterns. Male adopters did not differ significantly in CVD risk factors compared to inactive maintainers and relapsers. Among females adopting was associated with lower heart rate and total cholesterol compared to inactive maintaining. Active maintainers showed better mental health than inactive maintainers. Male active maintainers had an increased likelihood of good mental health compared to adopters. Female active maintainers reported greater satisfaction with life compared to adopters.

Conclusions The WHO HBSC questions seemed to be acceptable instruments to measure cardiorespiratory fitness. Answers from girls were more reliable and valid than the answers from boys. The IPAQ was not a valid instrument for adolescents.

Dissatisfaction with life, overweight and no active participation in sports were the strongest predictors for decreased physical activity during adolescence in boys. For girls, hazardous risk behaviors like smoking and alcohol use, low maternal education and maternal physical inactivity were the strongest determinants of decreased physical activity. A high parental level of education and physically active parents were associated with increased physical activity during adolescence.

Those who maintained their physical activity from adolescence to young adulthood had a significantly lower CVD risk profile and better mental health, compared to inactive maintainers. Compared to inactivity maintainers and to relapsers, adopting physical activity was not significantly associated with lowered CVD risk profile. Adopting physical activity between adolescence and young adulthood was not associated with lower mental distress